That thought led Brandon Johnson of the MIT and Timothy Bo wling of Purdue University in Indiana, to wonder how many other craters have vanished, either by erosion or by being swallowed into Earth's interior as its crust moves around, and therefore whether it is likely that some have surv ived and been overlooked. They have just published their analysis in Geology. Earth's crust formed more than 4 billion years ago, but the oldest surviving blocks of it large enough to harbor craters date back only 3.5 billion years. What is known of the sizes and orbits of modern asteroids suggests that, if things have not changed over the aeons , about 14 big asteroids (defined as having a diameter of more than 7.4km, which would cause a crater at least 85km in diameter) should hit Earth every billion years. That means 49, give or take seven, over the past 3.5 billion years. Such impacts may have been more common in the past, when more big asteroids were around. Allowing for this, Earth would have been hit by 113 , give or take 11, of them. Either way, a lot of craters are missing. Over the past 3.5 billion years, roughly how many asteroids may have hit Earth?